Kinetically Determined Hygroscopicity and Efflorescence of Sucrose

Aug 21, 2017 - Institute for Chemical Physics, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, People's...
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Kinetically Determined Hygroscopicity and Efflorescence of Sucrose−Ammonium Sulfate Aerosol Droplets under Lower Relative Humidity Lin-Na Wang,† Chen Cai,*,†,‡ and Yun-Hong Zhang*,† †

Institute for Chemical Physics, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ‡ Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *

ABSTRACT: Organic aerosols will likely form in semisolid, glassy, and high viscous state in the atmosphere, which show nonequilibrium kinetic characteristics at low relative humidity (RH) conditions. In this study, we applied optical tweezers to investigate the water transport in a sucrose/ (NH4)2SO4 droplet with high organic to inorganic mole ratio (OIR). The characteristic time ratio between the droplet radius and the RH was used to describe the water mass transfer difference dependent on RH. For OIR greater than 1:1 in sucrose/(NH4)2SO4 droplets, the characteristic time ratio at low RH (∼60%). We also coupled vacuum FTIR spectrometer and a high-speed photography to study the efflorescence process in sucrose/ (NH4)2SO4 droplets with low OIR. The crystalline fraction of (NH4)2SO4 was used to understand efflorescence behavior when the RH was linearly decreasing with a velocity of 1.2% RH min−1. Because of suppression of (NH4)2SO4 nucleation by addition of sucrose, the efflorescence relative humidity (ERH) of (NH4)2SO4 decrease from the range of ∼48.2% to ∼36.1% for pure (NH4)2SO4 droplets to from ∼44.7% to ∼25.4%, from ∼43.2% to ∼21.2%, and from ∼41.7% to ∼21.1% for the mixed droplets with OIR of 1:4, 1:3, and 1:2, respectively. No crystallization was observed when the OIR is higher than 1:1. Suppression of (NH4)2SO4 crystal growth was also observed under high viscous sucrose/(NH4)2SO4 droplets at lower RH.

1. INTRODUCTION Aerosols are ubiquitous in the atmosphere and influence the climate and public health. They can act as cloud condensation nuclei (CCN) or ice nuclei (IN). With absorbing air pollutants, aerosols decide the rate of heterogeneous reactions. Atmospheric aerosols also take multiphase processes in water uptake or release.1 In particular, recent attention has focused on the possibility that organic amorphous semisolids and glasses can exist as an important class of atmospheric droplets, specifically at low temperature and low RH conditions in the upper troposphere,2−4 which is related to the formation of cirrus cloud.5−9 High viscous state in aerosols results in the inhibition of mass transfer through the aerosol bulk and delays uptake and evaporation of water.2,10−13 Bulk phase diffusion, which is partially relevant to viscosity, is important to quantify the response of aerosols changing with conditions.14 Diffusion coefficients in solid or semisolid droplets have been estimated to be up to 7 orders of magnitude smaller than which in liquids.15,16 In order to determine the formation and properties of semisolid and glassy aerosols, it is important to establish methods to obtain the viscosity quantitatively or qualitatively.8,17,18 © XXXX American Chemical Society

Kinetic limitations on water transport for single-solute aqueous droplets in glass state or gel state were previously explored.19,20 Atmospheric aerosol droplets are typically complex mixtures of organic and inorganic species, and they may occur as semisolids or glass or as a mixture of both depending on their composition and ambient conditions.21,22 An improved understanding of organic/inorganic ternary aerosols is promptly needed to better understand the behavior and implications of these viscous droplets in the atmosphere. Water-soluble inorganic droplets in the troposphere are mostly composed of sulfate, nitrate, chloride, sodium, ammonium, and calcium ions.23,24 (NH4)2SO4 is the most abundant inorganic salt found in nonmarine tropospheric aerosols.25 In addition, organic compounds are usually mixed with inorganic compounds in a single droplet.26,27 Furthermore, it has been shown that highly viscous organic droplets exhibit retarded hygroscopic growth due to the kinetic limitation of water diffusion into the bulk of the droplet.28,29 While the hygroscopic behavior of (NH4)2SO4 has been well Received: June 6, 2017 Revised: July 27, 2017 Published: August 21, 2017 A

DOI: 10.1021/acs.jpcb.7b05551 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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WGM is a straightforward way to characterize the evaporation or condensation kinetics without requiring accurate determination of the droplet size.36 A simple exponential expression cannot perfectly describe the relaxation dynamics of glassy systems. Instead, the relaxation time scale for a system property can be well characterized by a function of the form given by the Kohlrausch−Williams−Watts (KWW) equation which is a stretched exponential function.36,37 When responding to an applied perturbation over certain time t, the temporal dependence of the response function, F(t), is given by38

characterized, there is still uncertainty in the mixtures of (NH4)2SO4 with glassy organics.21,30,31 For the mixed (NH4)2SO4 and sucrose system, Robinson et al. utilized a cavity ring-down aerosol extinction spectroscopy (CRD-AES) to study the water uptake properties at room temperature. They found that the optical growth depended on the (NH4)2SO4 weight-percent, which in turn controlled the phase state and ultimately determined the water uptake.32 Hodas et al. measured the hygroscopic growth factors (HGFs) of sucrose/(NH4)2SO4/H2O. A comparison of measured and modeled HGFs for the sucrose/(NH4)2SO4 droplets indicated the presence of a viscous semisolid phase, which inhibited the crystallization of (NH4)2SO4.33 While these studies mainly focused on the hygroscopicity, less attention was paid to the efflorescence kinetics and quantitative analysis of mass transfer in the ternary (NH4)2SO4 aerosols. With consideration of the complexity and peculiarity of sucrose/(NH4)2SO4 system, vacuum FTIR spectrometer and optical tweezers are adopted in this work to investigate physical processes at relative lower RH, i.e., the efflorescence of (NH4)2SO4 and water mass transfer in sucrose glassy states. From the spectral features in high resolution infrared spectra, we can accurately infer specific molecular interactions and formation of new species thus give sensitive characterization water content and phase state.34 On the other hand, in a typical optical tweezers measurement, single aerosol droplet smaller than 10 μm can be trapped and isolated for indefinite time scales. Thus, detail kinetics results are solely obtained from contribution of the tweezed droplet. Moreover, we can vary the gas phase and then monitor the evolving droplet as it responds to the environmental change. Then, we are able to infer the size, composition and refractive index of a single droplet, and characterize the fundamental properties influencing the droplet behavior such as mixing state and morphology.35 In detail, we explore the limitations of water transport in sucrose/ (NH4)2SO4/H2O ternary aerosol droplets with size of 4−8 μm through optical tweezers measurement over a wide range of RH. We employ an approach of coupling vacuum FTIR spectrometer and a high-speed photography to investigate efflorescence process with linear changes of surrounding RH. The methods can provide information about the ERH, crystalline fractions, morphology and crystal growth of (NH4)2SO4 in both (NH4)2SO4 binary droplets and sucrose/ (NH4)2SO4/H2O ternary droplets. By exploration of the variability in the hygroscopic behavior of the inorganic/organic droplets at various RHs in different mixing states from wellmixed liquids to amorphous solids or semisolids, this study can provide support for efflorescence behavior and water transfer limitation of the atmospheric aerosols under high viscous state.

F(t ) ≈ exp[−(t /τ )β ]

where τ is the characteristic relaxation time and β is a fitting parameter. β decreases markedly as the system approaches a glass transition. The KWW function expressed in a form that represents the relaxation of viscous aerosol toward an equilibrium state following a change in the gas phase conditions is needed. The response function takes the form: F(t) =

r (t ) − r (∞ ) r(0) − r(∞)

(3)

where r(t) is the evolving time response of the relaxing parameter, r(t) is the droplet radius at time t, r(0), and r(∞) are the initial and final values respectively; i.e., the response function represents the fractional progression in droplet size from the initial to final states. Equations 2 and 3 can be combined to give r(t ) ≈ r(∞) + (r(0) − r(∞)) exp[−(t /τ )β ]

(4)

The equation allows direct analysis of the kinetic profiles of the surrogate measure of size change.36 2.2. Definition of the Crystalline Fraction. Crystalline fraction, R, is defined as the ratio between the crystalline amount at a certain RH, MRH, and the total crystalline amount MT when all the liquid droplets are completely crystallized, R=

MRH × 100% MT

(5) −1

The absorbance of the ν-NH4 band (1414 cm ) is A0 before crystallization of (NH4)2SO4. When all the liquid droplets are completely crystallized, the absorbance of the νNH4+ band (1414 cm−1) is AT. At a certain RH, the absorbance of the ν-NH4+ band (1414 cm−1) is ARH. Then, (AT − A0) corresponds to the total crystalline amount (MT), and (ARH − A0) corresponds to the crystalline amount (MRH).39 Therefore, eq 5 can be rewritten as +

R=

2. METHOD 2.1. Water Transport Kinetics as Measured by Aerosol Optical Tweezers. The relative size change throughout the subsequent changes in RH can be estimated from19 Δr Δλ = r λ

(2)

ARH − A 0 × 100% AT − A 0

(6)

On the basis of eq 6, crystalline fraction (R) values of (NH4)2SO4 can be obtained at different RH values.

3. EXPERIMENTAL SECTION 3.1. Experimental Preparation. The analytical grade (NH4)2SO4 and sucrose were purchased from Beijing Chemical Reagent Company and used without further purification. The 0.1 mol L−1 (NH4)2SO4 solution was prepared by dissolving (NH4)2SO4 in triply distilled water. Sucrose/(NH4)2SO4 solutions with different OIRs were obtained by adding designed amount of sucrose into the (NH4)2SO4 solution.

(1)

in which r and λ represent the radius of the droplet and the wavelength of the whispering gallery modes (WGMs) used to track the size change, respectively. We use the expression to estimate the droplet size change, since a full Mie fit to a homogeneous sphere is no longer appropriate when the droplet form concentration gradients. The shift in the wavelength of a B

DOI: 10.1021/acs.jpcb.7b05551 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B 3.2. Experimental Technique. 3.2.1. Optical Tweezers. A detailed description of the device has been reported in previous publication.19 The optical tweezers was formed by focusing the beam (532 nm, 200 mW) from an argon-ion laser (MellesGriot 43 series) through an Olympus UIS2 PlanCN 100× oil immersion objective (1.25 N.A.). A blue LED centered at 455 nm was adopted as the illumination source for bright-field imaging of the trapped droplets received by a camera (Watec, 1/3 in., model 231S2). Droplets of 1 mol L−1 sucrose or sucrose/(NH4)2SO4 ternary solution were introduced into the sample cell and a single droplet from the cloud was captured. The environmental RH was controlled by changing the ratio of dry and humidified nitrogen gas flow. The flow rates were controlled by two independent mass flow controllers (Alicat Scientific No. 21-1-08-1-1-KM0410). The combined gas flow introduced into the cell was kept constant at 0.15 L min−1. The RH and the temperature of the combined gas flow were measured by RH-T detectors (Center 313) before the cell and in the outflow from the cell. The uncertainty of RH determination was ±2.5%. 3.2.2. Vacuum FTIR Spectrometer. A detailed description of the vacuum FTIR spectrometer has been previously reported elsewhere39 and thus is described briefly here. The vacuum FTIR spectrometer (Bruker VERTEX 80v) was equipped with a SiC MIR/FIR source (10,000−20 cm−1), a high precision 21° Michelson interferometer (Ultra Scan), 16 selectable mirror velocities (0.1−10 cm·s−1 opd) scanner, KBr on a gel beam splitter (range: 8000−350 cm−1), and liquid nitrogen cooled mercury−cadmium−telluride (MCT) detector. The pressures of vacuum optics bench and internal sample chamber were controlled by a single rotating impeller vacuum pump. CaF2 windows and rubber rings were used to seal the sample cell. The RH adjusted by the RH controlling system was determined by the water vapor absorbance from IR spectra. The experimental IR spectra were collected in the range of 1000− 4000 cm−1. Therefore, both water content within aerosols and water vapor amount of the aerosol ambient could be derived from the IR spectra. 3.2.3. High-Speed Photographic Investigations. An optical microscope (BMX, Shanghai) and a high-speed CMOS (complementary metal−oxide−semiconductor) video camera (MS55K, Mega Speed Corp., Canada) were used to track the morphological changes of droplets at a given RH. The schematic diagram of the experimental setup has been described previously.40 The high-speed CMOS camera was set at 25 frames per second. The RH was controlled by mixing the flow rates of water-saturated N2 and dry N2. The morphological changes of the droplets were recorded by the high-speed CMOS camera. The measurements were made at ambient temperatures of 295.15−297.15 K.

Figure 1. Radius (red line) and RH (black line) of an sucrose/ (NH4)2SO4 droplet (OIR = 2:1) during stepwise changes in the RH.

During this process, the response of size to the RH changes fast. A slowing in the droplet size change is observed when RH is below 25.0%, extending beyond the time scale of the actual change in RH. In this RH range, the droplet size responds slowly to the RH change and appears to never reach an equilibrium state. Similar sizes and RHs of pure sucrose aqueous droplets and ternary systems with different OIRs are included in Figure S1 in the Supporting Information. In this study, we use the logarithmic ratios of characteristic time36 for changes in the droplet size to the characteristic time for the change in RH, lg(τR/τRH), to evaluate the diffusion in viscous states at relatively lower RH. We have mapped the response of droplets between a large number of initial and final RH values, correlation between lg (τR/τRH) and RH shown in Figure 2. Points in the bottom-right half side of the plot

Figure 2. Ratio of characteristic time for response in size of droplet to characteristic time for RH probe response, mapped over a wide range in initial and final RHs for ternary mixtures of sucrose /(NH4)2SO4/ H2O (OIR = 2:1). The color denotes the logarithmic ratio of the characteristic time of the size response to the characteristic time of the RH probe response.

4. RESULTS AND DISCUSSION 4.1. Quantitative Analysis of Water Transport. To explore the water transport limitation in viscous droplets, aqueous droplets of sucrose and sucrose/(NH4)2SO4 (OIR = 2:1) are trapped by optical tweezers, the RH is changed in a sequence of steps, either by changing the RH in small steps (RH ∼ 10%), or through a large step (RH > 10%, e.g., from 63.7 to 35.5%). As shown in Figure 1, the size of the droplet changes through evaporation or condensation of water to get equilibrium with the changing RH. The droplet size decreases quickly as RH decreases from 65.7% down to 46.3%, and then the droplet size increases as RH increases back to 55.3%.

correspond to steps in decreasing RH, and points in the top-left half side correspond to steps in increasing RH. The logarithmic ratio approaches a value of 0 at high RH, which indicates free transport of water into and out of the droplet. For RH < 30%, the response time is typically 2 orders of magnitude bigger than that of a droplet at relatively higher RH. As RH decreases with increasing solute concentration in the droplet, bulk phase of the droplet becomes more viscous, with reduced diffusion coefficients and slower water transport. When both the initial C

DOI: 10.1021/acs.jpcb.7b05551 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B and the final RH are below 40%, the slowing of the mass transfer rate is obvious, which illustrates the kinetic limitation on water transfer. It is clear that inhibition of mass transfer occurs not only below the glass transition, but also above.11 Similar RH maps of the ternary systems with three different OIRs are included in Figure S2 in the Supporting Information. Characteristic times with varying dependencies on composition are derived and shown in Figure 3. The characteristic

Figure 4. FTIR spectra of (NH4)2SO4 droplets with linearly continuous decreasing of RH during the dehumidifying process.

appearance of the crystal (NH4)2SO4 feature at 1414 cm−1.34 Meanwhile, the absorbance of ν−OH band at 1641 cm−1 shows an obvious decrease, suggesting the evaporation of water from the droplets. The characteristic peaks of sucrose at 3500−2800 cm−1 and 1500−1000 cm−1 are attributed to a series of overlapping bands. The band at ∼3378 cm−1 assigns to the O−H stretch of water and sucrose. The bands at 1500−1000 cm−1 produced by the C−C−H deformation modes, C−O−H deformation modes, C−O stretching vibrations and C−C stretching vibrations, only show a small intensity difference with the decreasing RH.41 Therefore, the ν-NH4+ and δ−OH bands of sucrose/(NH4)2SO4 ternary droplets show similar trends to those of (NH4)2SO4 droplets in the dehumidifying process, The FTIR spectra of sucrose/(NH4)2SO4 ternary droplets (OIR = 1:4, 1:3, and 1:2) are shown in the Supporting Information (Figure S3). In the process of ν-NH4+ band shifting from ∼1441 to ∼1414 cm−1 for both (NH4)2SO4 binary system and sucrose/ (NH4)2SO4 ternary system, a marked isosbestic point at 1429 cm−1 is a clear indication for the phase transfer from liquid NH4+ to crystalline NH4+ in Figure 5. Similar phenomena for three different OIRs are shown in the Supporting Information (Figure S4). Then using these signatures described above,

Figure 3. Characteristic time for changes in size of droplets, different colors represent different RH steps (ΔRH). Pure sucrose and sucrose/ (NH4)2SO4 mixtures with OIR of 4:1, 2:1, and 1:1 are represented by filled squares, open triangles, filled circles, and open diamonds, respectively; the fit results are shown as a black short dash line.

time of radius of the sucrose binary droplets is between ∼77 and ∼1.5 × 104 s in an RH range of 60−25%, while, the characteristic time of radius is between ∼15 and ∼6.3 × 103 s, between ∼10 and ∼4.0 × 103 s and between ∼5 and ∼4.8 × 102 s for sucrose/(NH4)2SO4 droplets with OIR of 4:1, 2:1, and 1:1, respectively, at exactly the same RH range of 60−25%. Therefore, the characteristic time of the sucrose binary droplets is obviously higher than that of sucrose/(NH4)2SO4 ternary droplets at the same RH. As a result, diffusion constants of molecules in glassy organic and inorganic mixed droplets are strongly dependent on composition. The presence of different (NH4)2SO4 composition can lead to significant changes in the viscosity of the organic matrix. 4.2. The Crystallization Kinetics Investigated with the Vacuum FTIR Spectrometer. As already indicated, mass transfer for sucrose/(NH4)2SO4 droplets with high OIR have been made, clearly illustrating slow water diffusion in sucrose/ (NH4)2SO4 droplets at low RH. On the other hand, water mass transfer may influence the efflorescence of (NH4)2SO4 in sucrose/(NH4)2SO4 droplets with low OIR. Therefore, the efflorescence process is also important to further study in sucrose/(NH4)2SO4 droplets with low OIR. Figure 4 presents the in situ FTIR spectra of (NH4)2SO4 droplets with the linear decreasing RH with velocity of 1.2% RH min−1. For pure (NH4)2SO4 droplets, the bands at 3000−3600 cm−1 belong to the O−H stretching vibration (ν−OH) and the stretching vibration (ν-NH4+). Moreover, the bands at 1600−1700, 1385−1500, and 1000−1210 cm−1 belong to the O−H bending vibration (δ−OH), the bending vibration (ν-NH4+) and the asymmetric stretching vibration (ν3-SO42−), respectively. The characteristic of ν-NH4+ band becomes sharper and red-shift from 1441 to 1414 cm−1 as the RH decreases from 83.1% to 40.2%, the (NH4)2SO4 crystals can be known by the

Figure 5. Typical spectra of (NH4)2SO4 droplets during the crystallization process, the red, orange, olive, blue, and purple lines correspond to RH of 61.3%, 47.4%, 44.6%, 40.2%, and 19.9%, respectively. The absorbance of the solid NH4+ band at 1414 cm−1 can be used to calculate the crystalline fraction. D

DOI: 10.1021/acs.jpcb.7b05551 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B crystalline fraction R can be derived. Figure 6 shows time dependent crystalline fraction and the RHs in the dehumidify-

for sucrose/(NH4)2SO4 ternary systems with OIR of 1:4, 1:3, and 1:2, respectively. The decrease of ERH becomes more obvious with the increasing OIR. As shown in Figure 6, (NH4)2SO4 can completely crystallize in binary droplets, while there are 95%, 82%, and 73% crystalline (NH4)2SO4 formed for the sucrose/(NH4)2SO4 ternary droplets with OIR of 1:4, 1:3, and 1:2, respectively. This means that little (NH4)2SO4 solution remains at the minimal RH in the sucrose/(NH4)2SO4 ternary aerosols. A possible explanation is that organics suppress (NH4)2SO4 crystallization when droplets become viscous. Clearly, these results indicate that sucrose can significantly influence the crystallization of (NH4)2SO4, including the ERH and the crystalline fraction of (NH4)2SO4. The efflorescence process involves nucleation of the crystalline phase in a supersaturated liquid environment, which is followed by crystal growth and evaporation of water.42 In contrast to the phenomenon in (NH4)2SO4 binary droplets, there is a break point in the efflorescence process which is divided into two parts for the sucrose/(NH4)2SO4 ternary droplets. Such as sucrose/ (NH4)2SO4 ternary droplets (OIR = 1:3), the crystalline fraction of (NH4)2SO4 increases rapidly when RH deceases from 43.2% to 37.0% and starts to increase slowly when RH continues to decrease from 37.0% to 21.2% in the efflorescence process. Sucrose/(NH4)2SO4 ternary droplets with OIR of 1:2 and 1:4 show similar changes in the efflorescence process. This can be explained by the reason that crystal growth is suppressed at low RH when (NH4)2SO4 immerse in high viscous droplets. 4.3. Dynamic Images of (NH4)2SO4 Crystallization. High-speed photography allows resolving very fast processes occurring on the time resolution of subseconds such as efflorescence. Different from the vacuum FTIR method, highspeed photography can identify the corresponding morphology, such as initial nucleation site and (NH4)2SO4 crystal growth. Currently, it is not clear how the efflorescence process initiates

Figure 6. Crystalline fraction and RHs for (NH4)2SO4 droplets and sucrose/(NH4)2SO4 droplets as a function of time in the linear RH decreasing process (a velocity of 1.2% RH min−1). Results of pure (NH4)2SO4 and sucrose/(NH4)2SO4 mixture with OIR of 1:4, 1:3 and 1:2 are represented by navy, orange, violet, and olive solid lines, respectively. The filled area refers to the efflorescence RH range. The break points for the crystalline fraction vs RH were at 37%, 34%, and 32% for the OIR of 1:4, 1:3, and 1:2, respectively.

ing process for both (NH4)2SO4 and sucrose/(NH4)2SO4 aerosols. Crystallization occurs when the RH is ∼48.2% and almost accomplishes when the RH is ∼36.1% for the (NH4)2SO4 binary system, and the crystallization RH ranges are ∼44.7 to ∼25.4%, ∼43.2 to ∼21.2%, and ∼41.7 to ∼21.1%

Figure 7. Optical microscopy images of (NH4)2SO4 crystal growth for decreasing RH, The (NH4)2SO4 droplets are shown in row a, the sucrose/ (NH4)2SO4 droplets with OIR of 1:4, 1:3, and 1:2 are shown in rows b−d, respectively. Shown in the images are the relative humidity and time at which the images are recorded. The diameters of droplets range from 120 to 160 μm. E

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and proceeds in supersaturated viscous sucrose/(NH4)2SO4 ternary droplets. The initial crystal growth and the morphological changes of the droplet are shown by a series of pictures at different time in Figure 7. Time is set to zero for the frame before the initial crystal growth is observed. In Figure 7a, the initial crystal formation appears in the inner region of the (NH4)2SO4 droplet’s projection at 48.4% RH. Crystal growth proceeds until half of the droplet is covered at t = 0.04 s, then the droplet is fully covered with a crystalline coating at 0.08 s. Further morphological changes are observed as darker regions moving through the droplet. It has been reported previously that water molecules exist in effloresced droplets. Therefore, these regions correspond to water contained inside (NH4)2SO4 crystal. The last image of Figure 7a shows the morphology of the deeply dehydrated droplet at 37.4% RH (no morphological changes are observed after this time when the RH is further decreased). In Figure 7b, nucleation occurs at 48.0% RH. Crystal growth proceeds until the droplet is fully covered by the crystalline state at t = 0.8 s which is slower than pure (NH4)2SO4 droplet. The initial crystal has an x-shape which elongates in all four directions (the third panel of Figure 7b). Further changes occur much more slowly in the droplet. In contrast to the low viscosity of the pure (NH4)2SO4, which allowed instantaneous crystallization within 0.08 s, the crystalline state covers the whole mixed sucrose/(NH4)2SO4 ternary droplets with OIR of 1:4, 1:3, and 1:2 on the time scale of 0.8, 1.6, and 7.4 s, respectively. Moreover, sucrose/(NH4)2SO4 ternary droplets with OIR of 1:4 and 1:3 are aspheric and exhibit a degree of internal structure, suggesting that at least part of the droplet is in a crystalline state. The sucrose/(NH4)2SO4 ternary droplet with OIR of 1:2 exhibits a spherical morphology without any internal structure; it suggests that the droplet is in amorphous (semi)solid state.

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b05551. Two figures of radius and ratios of characteristic times of aqueous sucrose and sucrose/(NH4)2SO4 droplets (OIR = 4:1 and 1:1) during stepwise changes in the RH and FTIR spectra and typical spectra of sucrose/(NH4)2SO4 ternary droplets with OIR of 1:4, 1:3, and 1:2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.C.) E-mail: [email protected]. *(Y.-H.Z.) E-mail: [email protected]. Telephone: 86-1068913596. Fax: 86-10-68913596. ORCID

Yun-Hong Zhang: 0000-0002-2529-3538 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully appreciate financial support from the National Natural Science Foundation of China (Nos. 91544223, 41175119, 21473009, and 21373026) and the Ministry of Science and Technology of the People’s Republic of China (No. 2016YFC0203000).

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REFERENCES

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5. CONCLUSION Aqueous droplets of a number of organic substances tend to form semisolid, glassy and high viscous state as RH decreases. The dynamic nature of the hygroscopic response of the droplets to a change in RH depends on the diffusion of water molecules in the bulk phase of the droplet. For the ternary system of sucrose/(NH4)2SO4 droplets, mass transfer of water was found to be strongly dependent on sucrose to (NH4)2SO4 mole ratio, and the droplets showed nonequilibrium kinetic characteristics at low RH. The efflorescence process of (NH4)2SO4 in sucrose/(NH4)2SO4 ternary droplets was also found to be kinetically determined. When the OIR is 1:1, no crystallization was observed in the whole RH range for the ternary droplets, indicating that sucrose totally suppresses nucleation of (NH4 ) 2SO 4 in the droplets. Nucleation suppression capability increases with the increasing sucrose molar ratio. For the ternary droplets with OIR of 1:4, 1:3, and 1:2, initial ERHs of (NH4)2SO4 were observed at ∼44.7%, ∼43.2%, and ∼41.7% respectively, lower than that ∼48.2% for pure (NH4)2SO4. The crystal growth of (NH4)2SO4 was also suppressed at low RH in sucrose/(NH4)2SO4 ternary droplets (OIR = 1:2, 1:3, and 1:4), resulting in appearance of a break point on the curves of the crystalline fraction vs RH. Understanding water transport and efflorescence process in the sucrose/(NH4)2SO4 droplets is an imperative step toward studying the impact of semisolid or glassy organic aerosols on atmospheric chemistry. F

DOI: 10.1021/acs.jpcb.7b05551 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.7b05551 J. Phys. Chem. B XXXX, XXX, XXX−XXX